The majority of materials (metal alloys, ceramics, concretes, etc.) oxidise under the effect of an elevated temperature in the presence of a gaseous atmosphere. This phenomenon of oxidation is amplified and becomes damaging when the material is subjected to considerable cyclic variations in temperature: during the first cycles, exposure of the material to high temperatures causes the formation of a protective oxide layer (transitory oxidation phase), which grows until it reaches a critical thickness above which any cooling to which the material is subjected causes flaking of the protective oxide layer; during subsequent cycles, cooling operations to which the material is subjected cause the protective oxide layer to break up, and successive oxidations at high temperature of the surface of the material thus exposed deplete it of elements permitting the formation of the protective oxide, to exhaustion; cyclic exposure of the material to high temperatures then manifests itself as in-depth oxidation of the material (formation of metal oxide sub-layers, which flake during the cooling phases), which thus attacks the material until it breaks.
In addition to oxidation of the material, exposure of a material to high temperatures can also cause phenomena of corrosion, passivation or adsorption, which manifest themselves as a gain in mass of the material, or phenomena of decomposition, dehydration, pyrolysis, combustion or dehydroxylation, which manifest themselves as a loss in mass of the material. These phenomena can be observed and studied by thermogravimetry.
The thermogravimetric study of the behaviour of a material subjected, in the presence of a gaseous atmosphere, to considerable variations in temperature is fundamental to many industries: aviation (gas turbines, especially of aircraft engines), the automotive industry (exhaust pipes, catalytic converters, etc.), chemical engineering (chemical and petrochemical factory reactors), the nuclear industry, the electrical industry (thermal generators), etc. It allows evaluation of, inter alia, the resistance of a material to thermochemical attacks, its lifetime, the risks of cracking, the maximum possible use temperature of the material, etc., and makes it possible to work at developing new, higher-performance materials which are capable of withstanding higher use temperatures. The thermogravimetric study of the behaviour of a material at high temperature is also of interest ecologically: increasing the use temperatures of materials results in a greater efficiency of the industrial processes in which the materials are involved, and a subsequent reduction in energy consumption, CO2 emissions, etc.
During use, materials undergo attacks of various origins: attacks of thermal and/or chemical origin (high-temperature oxidation, cyclic oxidation, corrosion, decomposition, dehydration, etc.), mechanical stresses, cyclic thermo-mechanical fatigue, etc. These various factors interact in a complex manner. And there is at present no laboratory test that is capable of reproducing all these factors at reasonable cost and in reduced times.
For a material subjected to elevated temperatures (from 400 to 1800° C.) and/or to considerable variations in temperature, thermal phenomena (and especially the phenomena of oxidation and/or corrosion at high temperature, depending on the composition of the gaseous atmosphere) often prove to be the most important. For that reason, the lifetime of a material subjected to such use conditions is evaluated on the one hand by means of simplified and “accelerated” thermogravimetric tests, which allow measurement of the effects of exposure (isothermal or cyclic) of the material to high temperatures for a given period of time, and on the other hand by means of mathematical simulation models which, when applied to the experimental results of the preceding thermogravimetric tests, make it possible to simulate and extrapolate not only the long-term action of cyclic high-temperature exposure (as a function of the actual use conditions of the material—chemical composition of the atmosphere, maximum temperature of the material, time of exposure to high temperature for each cycle, number of cycles, rate of heating, rate of cooling, etc.—which are often different from the test conditions), but also the effects of other possible factors (mechanical, thermo-mechanical, etc.), the interaction of the various factors, the random nature of certain phenomena, a statistical modulation of the experimental results.